Gardening Zones and Frost Dates

One of the most common questions I receive from fellow gardeners is "What hardiness zone do I live in and what the heck is a hardiness zone anyway?"

Well, it is an important question in helping decide what plants to use in your garden. You see, all plants are cold tolerant to a certain temperature and unless you plan on treating a plant like an annual, meaning that it will only live one growing season, you need to know if it will survive through the winters in your area. This is especially important with big-ticket items like trees, shrubs and roses.

To help gardeners with this information the U.S. Department of Agriculture developed the Plant Hardiness Zone Map. This map divides the continent of North America into 11 regions or zones. The average coldest temperature for that region determines each zone. The coldest zone is zone 1, which includes areas such as the extreme northern parts of Canada, parts of the Yukon Territory, and the Alaskan interior. Average lows are about – minus 50 degrees F in zone 1 (now that's cold!). Zone 11 is the warmest zone with average low temperatures being 40 degrees F. The other zones fall in between these extremes. What this means is that if you buy a plant that is cold tolerant to zone 7 and you live in zone 5, that plant is not going to make it through the winter.

In addition to the Plant Hardiness Zone Map, many people also reference the American Horticulture Society's Heat Zone Map, which helps you determine if a plant will survive the summer weather in your area. The AHS explains the map this way:

"The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days)."

With the aid of the hardiness and heat zone information you can select plants that are right for your climate, but it is also important to know when it is safe to put plants out in order to protect them from a late freeze or to allow them enough time to get established before winter sets in. For guidance you can use the average first and last frost dates in your area. The first frost date occurs in fall and the last frost date is in spring. These dates will determine your growing season.

There are also various ways of describing a frost. Usually the first frosts of the season are light (above 32 degrees F) and the amount of damage caused depends on the duration of the cold temperatures. If the mercury hovers around the mid 30s and dips to the freezing point for a few hours, many cold hardy plants like chrysanthemums can take it without skipping a beat. In the spring, after I have put out my lettuce, broccoli and other cool season crops, I am sometimes surprised by a late frost. I know the damage will be minimal if the weather warms up the next day. However if a blackberry winter sets in and the temperatures stay cold for several days, I know I must protect these plants with a cold frame.

Now, there are also light, moderate and hard freezes. A light freeze occurs when temperatures hover between 29 – 32 degrees F. This is what usually kills all my summer annuals in late fall, but many of my perennials and shrubs will be unaffected. Again, the damage will depend on the duration of the freeze. And as you might expect, colder temperatures means more widespread damage. A moderate freeze is considered anything between 25 – 28 degrees F. And a hard freeze is anything below 24 degrees F. While this seems like a lot of detail this is good information to know when it comes to protecting your plants during cold weather.